1. Field of the Invention
The present invention relates to a radar apparatus equipped with an abnormality detection function, and more particularly to an FM-CW radar apparatus equipped with a noise peak detection function.
2. Description of the Related Art
An FM-CW radar radiates forward transmitted waves frequency-modulated by a modulating signal of triangular waveform alternating cyclically between an upsweep section and a downsweep section, and produces a beat signal by mixing the waves reflected from a target with a portion of the transmitted waves. When the frequency of the beat signal in the downsweep section is denoted by fb(down) and the frequency of the beat signal in the upsweep section by fb(up), the beat frequency fr due to the distance to the target and the beat frequency fd due to the relative velocity of the target are respectively calculated asfr=(fb(down)+fb(up))/2  (1)fd=(fb(down)−fb(up))/2  (2)From these, the distance R to the target and the relative velocity V of the target can be respectively calculated asR=c·frT/4ΔF  (3)V=c·fd·/2f0  (4)(where c is the velocity of light, T is the period of the triangular wave, ΔF is the frequency modulation width (frequency shift width), and f0 is the center frequency.) Therefore, the values of fb(down) and fb(up) associated with each target are determined from the peaks appearing in the beat signal spectrum in the frequency domain obtained by Fourier transforming the beat signal, and the distance and the relative velocity of the target are determined using the equations (1) to (4).
In FM-CW radars, if the output or input/output characteristics of devices such as oscillators, mixers, etc. have frequency characteristics (frequency dependence), the transmit wave and the local signal are AM modulated due to the FM modulation of the transmit wave, and FM-AM conversion noise occurs. If this FM-AM conversion noise is contained in the beat signal, a noise peak appears in the Fourier transform result in addition to the peaks related to fb(down) and fb(up) described above. This can degrade signal detection accuracy and can cause false recognition. Accordingly, some means must be provided to detect noise peaks including the peak due to the FM-AM conversion noise.